The present invention concerns a method and apparatus for producing complex carbon molecules and, in particular, a method and apparatus that utilizes the plasma within an inertial electrostatic confinement (IEC) device to convert a carbon-based gas into "buckey-balls" or fullerene C.sub.60 and sister molecules.
The IEC was originally developed as a neutron source for activation analysis as reported in G. H. Miley, J. B. Javedani, R. Nebel, J. Nadler, Y. Gu, A. J. Satsangi, and P. Heck, "An Inertial Electrostatic Confinement Neutron/Proton Source," Third International Conference on Dense Z-pinches, eds. Malcom Haines and Andrew Knight, AIP Conference Proceeding No. 299, AIP Press, New York, 675-689 (1994). For such application, when a gas is introduced into the chamber in the tens of mTorr pressure range, a plasma discharge is created by applying high voltage (10-100 kV) to the grid. The grid also serves to extract ions from the discharge and accelerate them toward the center of the device, where a dense, high-temperature plasma is formed. The potential surfaces are shaped such that ions are trapped and recirculated, creating a highly non-thermal plasma with energetic (kV) ions and lower-energy background electrons. The resulting plasma provides several unique opportunities for plasma processing, either using in situ methods or employing radiation emitted from the dense core region.
An inertial electrostatic confinement (IEC) particle generator is described in U.S. patent application Ser. No. 08/232,764 (Miley et al.) which was filed on Apr. 25, 1994 and is incorporated herein by reference. The inertial electrostatic confinement device disclosed therein includes a metallic vacuum vessel which is held at ground potential and contains internally and concentric to the vessel, a wire grid which acts as a cathode. The cathode may be made from a variety of metals having structural strength and appropriate secondary electron and thermionic electron coefficients. The cathode wire grid is connected to a power source to provide a high negative potential (30 kV-70 kV), while the vessel itself is conductive and maintained at a ground potential. Deuterium or a mixture of deuterium and tritium gas is introduced into the vessel. A voltage is applied to the cathode wire grid and the pressure is adjusted in order to initiate a glow discharge. To maximize the neutron yield per unit power input while maximizing grid life-time by reducing collisions with a grid, operational conditions are used to create a "star" glow discharge mode. The glow discharge generates ions which are extracted from the discharge by the electric field created by the cathode grid. These ions are accelerated through the grid openings and focused at a spot in the center of the spherical device. The resulting high energy ions interact with the background gas (beam-background collisions) and themselves (beam-beam collisions) in a small volume around the center spot, resulting in a high rate of fusion reactions. The result is a neutron generator producing neutrons as one of the D-D or D-T fusion reaction products. Where the extraction rates are high, the extracted ions may provide a deep-self generated potential well that confines trapped beam ions, creating even higher reaction rates. The device may be modified by using a field gas mixture of deuterium and helium-3 to be a source of protons rather than neutrons. One geometrical form of the device is spherical and is seen in FIG. 1. This device is based upon the principle of an ion accelerator with a plasma target. In a neutron-generator embodiment, deuterium-deuterium fusion reactions take place in the plasma target zone and produce energetic neutrons. The device acts as a simple spherical plasma diode, having a ground potential on the outer sphere and a negative potential on a nearly geometrically transparent inner spherical grid. The spherical inertial electrostatic confinement device 10 is illustrated in FIG. 1 where a conductive vacuum chamber 11 is connected to a ground potential at contact 17. The device has a cathode grid 12 that defines a small sphere within the chamber and has a grid design that provides a high geometric transparency. In operation, however, this grid design has an even higher effective ion transparency, due to the effect of a concentration of ions into "microchannels", as subsequently described. A source of power 14 is connected by a high voltage feed-through to the internal cathode grid 12. The voltage has a negative value, thereby providing a bias between the relatively positive walls of the vacuum chamber and the central grid area. Gas is introduced into the vacuum chamber 11 by a control valve 15 and is evacuated by a pump 18, providing a means of controlling the gas pressure in the chamber.
Upon application of a potential to the cathode grid, under certain grid-voltage, gas pressure, gas type and grid-configuration conditions, high density ions and electron beams will form within the IEC device initiating a "star" mode of operation. In this mode, high density space charged neutralized ion beams are formed into microchannels that pass through the open spaces between the grid wires. As the ions avoid contact with the wires, this mode increases the effective grid transparency to a level above the geometric value. These microchannels significantly reduce grid bombardment and erosion and increase power efficiency. For conventional star mode operation, the grid and microchannel beams are symmetric so that a convergent high-density core develops. The inertial electrostatic confinement device serves as a valuable source of neutrons or protons.
Non-thermal plasma production in the IEC leads to several other quite different but possible applications. One that has been explored to date is the production of ultraviolet (UV) radiation. The device provides a high-intensity UV-radiation source if heavy gases, such as krypton or xenon, are used. Another application is the use of the IEC to create thrust by flowing the plasma out through a channel created by an enlarged grid wire opening. A process chamber using a quartz window to contain the flowing fluid under treatment has been designed and both of the foregoing applications are disclosed in a provisional application Ser. No. 60/030,009 filed on Nov. 1, 1996 and entitled Ion Jet Thruster Using Inertial Electrostatic Confinement Discharge Plasma, and PCT Application No. PCT/US97/19306; filed on Oct. 31, 1997 and entitled "Plasma Jet Source Using an Inertial Electronstatic Confinement Discharge Plasma", which are incorporated herein by reference. The application of the IEC structure to the production of fullerene also has been explored.
Carbon-60 was discovered in 1985 and was found to have three-dimensional, cage-like, all-carbon molecules in a gas phase carbon cluster. These even-numbered soccerball-shaped robust molecules were named "fullerenes" after R. Buckminster Fuller, the American architect who pioneered geodesic design. Since that time, there have been only a limited number of studies and papers presented on the subject of fullerene production and theory due to the relative unavailability of the all-carbon materials. Nonetheless, it was also found that in addition to the originally identified carbon-60 and carbon-70, there were hosts of other stable carbon configurations ranging from carbon-24 up to carbon-240 and beyond. Moreover, within the past 5 years, there have been modest strides in the production of carbon-60 and carbon-70 and limited yields of the higher and lower order carbon molecules.
Recently, the demand for fullerenes has been growing due to their potential applications. Many advanced materials currently in use show only a single application, but fullerenes show a series of applications, which include their use as superconductors, anti-AIDS drugs, catalysts and catalyst supports, photoconductors, optical limiters, adsorbents, precursors to synthetic diamonds, and plant growth regulators. Additionally, a major thrust of fullerene research is to exploit its use for energy production. Recent studies show that carbon-60 is a good hydrogen storage medium and can attach more hydrogen atoms (up to 48) per single storage molecule as compared with conventionally used storage material like palladium. Another area that is related to energy production is the use of carbon-60 as battery electrodes. Fullerene-based electrodes would be light in weight and comparable with conventional nickel-oxide electrodes in efficiency. Finally, Carbon-60 has also been thought of as an excellent candidate for many new applications in the near future, such as molecular ball bearings for ships and as a propellant for electric thrusters on satellites. By far the most advanced concept is in the realm of microstructures-the nanotube-wherein an all-carbon linked structure that is completely cylindrical and tubular, can have metallic and semiconductor properties.
Production of fullerene to achieve these results has been approached on both a theoretical and practical level. For example, the method of formation of the fullerene carbon molecules has been subject to several theories. One theory is that graphite exists in the form of sheets that are made up of pentagons and hexagons and, as a result of a physical tendency for such structures to gravitate toward the lowest energy levels, bend to eliminate their highly energetic dangling bonds, present at the edges of the growing structure following the "Pentagon road rule" discussed by Smalley (Smalley, R. E.; "Self-assembly of the fullerenes" Acc.Chem.Res 25:98-105, 1992). Closure of these bent or curled graphite sheets results in the formation of a closed spheroidal cage of carbon atoms. (Zhang et al. "Reactivity of large carbon clusters: Spheroidal carbon shells and their possible relevance to the formation and morphology of soot"; J. Phys. Chem. 90-525, 1986).
Fullerene was first produced in the hot carbon plasma generated during laser ablation of graphite by time-of-flight mass spectroscopy. Since then there have been several other attempts at fullerene production. In 1990, Kratschner et al ("Solid C-60: A new form of carbon"; Nature 347:354-358, 1990) disclosed a simple method for the production of macroscopic quantities of carbon-60 by resistive heating of graphite in an inert atmosphere. Since that time, this method has been continuously improved by subsequent changes in reactor size, rate of graphite rod consumption, and helium pressure. U.S. Pat. No. 5,534,232 teaches the introduction of carbon halides into a plasma torch, which disassociates molecules into carbon and halogen atoms, forming a carbon cloud that condenses into a soot containing fullerenes. However, the halogen atoms can enter into the condensation process, preventing the formation of C-60 in some instances, thus reducing the overall production efficiency. Also, in that case, there is no use of a potential field and the plasma in the torch is Maxwellian. Japanese published application No. 61-73891 also concerns the use of a plasma to produce fullerenes, but does not use a potential field in the separation process. Other patents, including U.S. Pat. Nos. 5,510,098, 5,316,636, 5,494,558 and 5,395,496, use various processes to vaporize carbon rods, producing carbon atoms that recombine into fullerenes.
For larger-scale production, Peters et al ("A new fullerene synthesis" Agnew. Chem.Int.Ed.Engl. 31:223-224, 1992) developed a thermal vaporization technique using a high-frequency oven which gave modest yields of carbon-60 at temperatures of 2700C, and this approach has been extended to the vaporization of graphite using intense sunlight. Other vaporization techniques involved arcing either by brief contact of conducting graphite rods or by means of a plasma discharge (Parker et al., "High yield synthesis, separation and mass spectrometric characterization of fullerene C60-C266", J.Am.Chem.Soc. 113:7499-7503, 1991). A direct current furnace has been used to give highly enriched carbon-70 and significant amounts of other-order carbon molecules have been produced by electron beam evaporation processes. To date, however, none of these approaches have demonstrated sufficient efficiency so as to be considered economically attractive.
In addition to using graphite as the host material, coal has been used as the starting material with a laser evaporation process then used to start a whole new regime of carbon formation using any carbonaceous material. Formation through the combustion or pyrolysis of aromatic hydrocarbons like benzene, as disclosed by Taylor et al. "Formation of C60 by pyrolysis of naphthalene", Nature 366:728-731, 1993).
The production of fullerene in the foregoing setups has been limited in quantity of material and efficiency. Of the "soot" that is collected, it is comprised of all of the reformed carbon from the host material that was consumed. Fullerenes are produced in addition to hydrocarbons, reformed graphite and carbon oxides. Soot production levels of a few grams per hour to hundreds have been achieved with efficiencies of 50% to &lt;1%. In best case scenarios, amounts of approximately 2 grams of carbon-60 material per hour are produced in a plasma arc reactor (Anderson et al., "A plasma arc reactor for fullerene research"; Rev.Sci.Instrum, 65(12):3820-3822, 1994).
Although the foregoing production techniques have allowed the scientific community greater access to the carbon molecules, a need for highly efficient methods with a reasonable production rate for economic manufacture of quality fullerene substances still remains. Thus, the full utilization of the originally identified carbon-60 structure (fullerene) and its sister molecules will not be economically feasible for large scale applications unless a suitable method of production evolves.
Accordingly, it is an object of the present invention to utilize an energetic non-thermal plasma discharge as a medium for fullerene (C-60) production.
It is yet another object of the present invention to utilize the Inertial Electrostatic Confinement (IEC) device for the non-thermal production of fullerene (C-60) and its sister molecules, and to take advantage of its strengths and uniqueness over other forms of production.
Also it is a further object of the present invention to provide a method suitable for efficient production of fullerene (C-60) and its sister molecules, potentially on a commercially viable scale, utilizing a relatively simple but efficient device and process.